Photoelectronic image detecting devices



United States Patent U.S. Cl. 250-213 3 Claims ABSTRACT OF THEDISCLOSURE An image intensifier tube having a cathode window covered bya highly insulating transparent material such as silica, and wherein thetube may also have an insulating casing. By insulating the tube in thisway leakage currents through the tube which dispense as corona arereduced, especially at the cathode window, and consequently anyparasitic background to the amplified image provided by the tube is alsoreduced.

This invention relates to electron image intensifier tubes. It isparticularly concerned with tubes of the kind comprising a photo-cathodearranged to receive a light image and provide an electron beamdistribution which is amplified within the tube to impinge upon the endwindow.

In the operation of modern image intensifier tubes it is usuallynecessary to apply a high potential of the order of 20 to 50 kv.,between the cathode (input) and anode (output), the former beingnegative relative to the latter. It is also preferred in many cases tomaintain the cathode at a high negative potential and the anode at earthas observations or recording must be done at the output and this can beinconvenient, if not dangerous, if this end of the tube is at say +50kv.

However, the advantage of operating with the input (cathode) end of thetube at 50 kv. and the' output at earth is offset by the fact thatspurious background in the recorded image is usually much worse. This isdue to the fact that the high potential applied to the cathode insidethe glass end window of the tube produces an electrical leak throughthis glass which charges up to a high potential on its outer surface. Itwould ultimately reach the same potential as the cathode but if this is-20 to 50 kv. there will be a tendency for the charges on the externalsurface of the glass window to disperse as a corona discharge into thesurrounding air. This corona discharge can be substantially suppressedfrom the cylindrical body of the tube by encapsulating it in a jacket ofhighly insulating material such as silastomer, a highly insulatingrubber compounds. However, this cannot be used to cover the cathodewindow since the optical image must be projected through it onto thephoto-cathode. It is found that much undesirable background has itsorigin in the light from corona discharge from the outer surface of thisglass Window, the light being produced in a place from which it readilyreaches the cathode. This effect is particularly bad in the Spectraconor Lenard window image recording tube since, for various reasons, thewindow in this case is preferably made of lime-soda glass ofcomparatively low resistivity. The Spectracon is an image recording tubewith a photo-cathode adjacent to a cathode window, and a thin micaoutput window or Lenard window, allowing electrons to pass outside thetube to an electron sensitive emulsion. It turns out that leakagecurrent of the order of several microamps must be dissipated from thecathode window to the nearest object at earth potential. A dischargecurrent of this mag- 3,470,3801 Patented Sept. 30, 1969 nitude issufficient to produce enough light to cause appreciable photoemissionfrom the cathode and hence serious parasitic background to the recordedimage. This can be prohibitive if such a tube is being used to recordoptical images so faint that exposures of many hours are required.

It is an object of the invention to provide a tube in which the effectsof background light due to the high negative potential of the cathode isreduced as far as possible.

According to the present invention in an electron image intensifier tubeof the kind comprising a photo-cathode arranged to receive a light imageand provide an electron beam distribution which is amplified within thetube to impinge upon an end window at earth potential, the face of thetube adapted to receive light image is shielded by means of a lighttransparent plate of insulating material.

Preferably the plate is formed of silica.

In order that the invention may be more fully understood reference willnow be made to the accompanying drawing in which:

FIGURE 1 illustrates an embodiment thereof in side elevation, and,

FIGURE 2 is an end view of the tube illustrated in FIGURE 1.

Referring now to FIGURE 1 there is shown therein an image intensifiercomprising a cylindrical tube 1 of soda glass or similar material to oneend plate 8 of which there is secured a photo-cathode 2 and the otherend plate 6 includes a mica window 3. A succession of ring electrodes 4are provided spaced apart along the length of the tube and a potentialdivider chain 5 is connected to the electrodes 4. The end plate 6 of thetube containing the mica window 3 is designed to be held at earthpotential and an insulated e.h.t. lead 7 at a potential of -40 kv. isconnected to the end plate 8 to which the photo-cathode is attached.

In order to reduce effects of corona discharge from the end plate 8 afurther plate 9 is cemented to the external surface of end plate 8.Plate 9 is formed of silica a few millimetres thick and since it has aresistivity of between 10 and 5x10 the resistivity of soda glass, theleakage current and hence the corona discharge is reduced byapproximately these factors.

To still further reduce electrical leakage into the air a casing 10 ofPerspex, a poly-methyl methacrylate, is provided surrounding the tubeexcept for a slot 11 which is provided in the front end face of casing10 to allow impingement of an optical image on photo-cathode 2. It isalso convenient to bring the e.h.t. lead 7 along the length of the tubecasing 10 and out of the tube adjacent the rear end window 6 in order toreduce the effects of discharge from lead 7 in the neighbourhood of thephotocathode.

The background can be further reduced by coating the external surface ofthe silica plate 9 with a conducting layer of, say, metal or Aquadag,leaving free only that area through which the light must enter. This isfor example a window 10 x 30 mm. in the case of the Spectracon. Thiswindow can also be surrounded by an opaque mask or hood projecting somedistance from the surface of the window, say 2 cm. in the case of theSpectracon. A small foil, say 1 cm. x 1 cm., of metal coated with a lowintensity source of aor B-ray (say, strontium 90, about 1;; curie ofB-rays) emitting radioactive substance is fixed to the conductingcoating on the silica plate and is preferably placed near its outeredge. The SI-radiation from this source will produce ionization of theair in its immediate neighbourhood so that as the potential of theconducting coating of the silica begins to rise due to the minuteelectrical leak through it these ions will discharge it before itspotential can reach a level at which it can produce uncontrolled coronadischarges into the air. Any small light emission in the neighbourhoodof the radioactive foil will be screened from the photocathode by thehood surrounding it.

To summarize, the electrical leakage that produces serious background bycausing corona discharge in the neighbourhood of the cathode is greatlyreduced by the interposition of a silica plate and any residual chargeis disposed of by a controlled ionization discharge.

1 claim:

1. An electron image intensifier tube, including:

an envelope which is transparent in at least one area;

a photo-cathode arranged to receive a light image through the said areaand to provide an electron beam distribution;

supply means for supplying the photo-cathode with a highly negativevoltage of the order of tens of kilovolts; and

a light transparent plate of highly insulating material compared withthe material of said area positioned in the path of light received bysaid photo-cathode,

UNITED STATES PATENTS 2,030,491 2/1936 Abadie 313--312 X 2,120,9166/1938 Bitner 250-213 X 2,145,727 1/1939 Lloyd 313-58 X 2,833,953 5/1958Rogers 313313 X FOREIGN PATENTS 685,725 1/1953 Great Britain.

JAMES W. LAWRENCE, Primary Examiner RAYMOND F. HOSSFELD, AssistantExaminer US. Cl. X.R. 313-102, 312, 313

